1. Field of the Invention
This invention relates to a liquid crystal alignment agent, more particularly to a liquid crystal alignment agent having improved alignment properties. The invention also relates to a liquid crystal alignment film formed from the liquid crystal alignment agent, and a liquid crystal display element including the liquid crystal alignment film.
2. Description of the Related Art
A liquid crystal display element is widely used in a screen of a computer, a viewfinder of a camera, a projection display, a television screen, or the like.
Nematic liquid crystal display elements are predominantly used in general liquid crystal display elements, and concrete examples of the nematic liquid crystal display elements actually used include: (1) a TN (Twisted Nematic) liquid crystal display element, in which a liquid crystal alignment direction of one side substrate is twisted at a 90 degree angle relative to a liquid crystal alignment direction of the other side substrate; (2) a STN (Super Twisted Nematic) liquid crystal display element, in which a liquid crystal alignment direction of one side substrate is twisted at an angle greater than 180 degrees relative to a liquid crystal alignment direction of the other side substrate; and (3) a TFT (Thin Film Transistor) liquid crystal display element which uses a thin film transistor.
It has been continuously attempted in the art to improve a liquid crystal alignment film in order to enhance the display performance of the liquid crystal display elements.
The liquid crystal alignment film is made of a liquid crystal alignment agent. Conventionally, the liquid crystal alignment agent is formulated by dissolving polyamic acid or soluble polyimide in an organic solvent, and is then applied and cured on a substrate to form the liquid crystal alignment film. Although the liquid crystal alignment film formed from the conventional liquid crystal alignment agent formulated from polyamic acid or polyimide has good heat resistance, stability of the pretilt angle thereof is not satisfactory.
JP 07-110484 (A) discloses a liquid crystal alignment film, which contains a polymer having a repeating unit expressed by a formula recited therein, and which is used to obtain a stable large pretilt angle and excellent orienting characteristics of a liquid crystal and to obtain good electro-optic characteristics.
JP 07-043725 (A) provides an oriented film which has a stable and high pretilt angle and good electro-optical characteristics by containing a polymer having a specific repeating unit defined therein.
However, in addition to the expected polymer such as polyaspartamide, there is also contained a relatively large amount of residual reactants in the product. Therefore, the liquid crystal alignment agent made from the polymer may have a problem of inferior storage stability.
It is still required in the art to develop a liquid crystal alignment agent which can effectively reduce an image sticking phenomenon and which has improved storage stability.